What is a paper engineer? Something like an architect—but for paper. (Think pop-up books and origami, not bridges and skyscrapers.)
The Pocket Paper Engineer, a classic text that gives step-by-step instructions for making your own pop-up books, is available for free on Google Books. And if you're thinking of encouraging (or engendering) a little person's interest in all things architectural, there's no better way to show them the connection between careful planning, precision of execution, and a satisfying end result than with a great pop-up book.
A few pop-up titles to get you started:
Frank Gehry in Pop-Up. Love him or hate him, you gotta admit that if any architect's oeuvre lent itself to be popped-up—it's Gehry’s.
600 Black Spots: A Pop-up Book for Children of All Ages. Author David A. Carter has a franchise going with his high-art abstract pop-ups. Though his books are advertised "for children of all ages," note that kids under 3 will quickly denude the book of all 600 black spots, then eat and/or hide them.
ABC3D. Another high-art pop-up that takes a refreshingly abstract approach to teaching the alphabet. Cookie Monster would plotz.
Star Wars: A Pop-Up Guide to the Galaxy. Five words: Pop-up Jabba the Hut. (And 172 positive Amazon reviews can't be wrong.)
Hard to find (it's not published in the U.S.) but thrilling for architecturally-inclined kids is Robert Crowther's Deep Down Underground, a jauntily illustrated, often politically incorrect English pull-tab and pop-up book that combines "What's under that rock?" fascination with paper-engineering geekery. Each page features a different subterranean part of the U.K. landscape—from bat-infested caves and rat-infested garbage dumps to leaky offshore drilling platforms. And unlike every other pop-up book, Deep Down Underground doesn't hide the inner workings of its paper magic: each illustrated page is followed by a behind-the-scenes look into its paper tabs, levers, and fulcrums. The normally glued-together pages are flayed open, to reveal illustrations in mark-up blue ink on graph paper, that show exactly how each "trick" works, with explanations from the hard-hatted cartoon paper engineers—and paper construction workers—themselves.